A primary objective of electromagnetic launcher design is to maximize efficiency while minimizing the complexity, mass and expense of the structure. In the context of electromagnetic launch technology, efficiency may be defined in terms of the relative amount of kinetic energy supplied to a projectile compared to the energy supplied by a source to launch the projectile. A more efficient rail gun design will normally require a smaller, lighter and less expensive energy source than a less efficient rail gun. Weight considerations for the total system comprised of both the rail gun and its source are particularly important for launcher systems that are intended to be orbited in space.
Several rail gun designs have been developed which have different efficiencies and degrees of complexity. The simplest rail gun system consists of two conducting rails which carry current to a conducting element or plasma at the rear of a projectile. The efficiency of this simple type of rail gun system is very poor, generally in the range of ten to thirty percent. More advanced concepts include self-augmenting rails, for example as shown in U.S. Pat. No. 4,347,463 to Kemeny, et al., segmented distributed energy-storage rails, and non-segmented distributed energy storage rails. These more complex systems are capable of achieving energy efficiencies in the forty to sixty percent range.
There are three main sources of energy loss in an electromagnetic launcher system the resistive losses in the firing rails, the loss in the plasma behind the projectile, and the loss of the energy stored in the magnetic field from the rail current, which is typically dissipated in muzzle resistors. The relative efficiency of a rail gun at a particular level of firing currents applied to the rails can be improved by increasing the magnetic field through which the projectile moves. Augmenting coils connected in series with the firing rails have been proposed and would carry millisecond pulses of the very large currents which are required by the firing rails. The augmenting coils are of normal metal conductors, typically copper or aluminum, and thus resistively dissipate some of the energy from the source.
Superconductive coils cannot be used for augmenting coils because the pulsed firing currents would drive the superconductive coils into the nonsuperconducting normal state. It has also not been feasible to use superconductive coils driven with constant current which would serve to provide the persistent magnetic field through which the projectile could be driven. The pulsed armature current passing through the firing rails and the plasma between them create magnetic field pulses, which, if applied to the superconducting coil, would drive the superconductor to a normal conducting state. Although superconducting coils could theoretically be shielded from the time varying magnetic fields, the size, weight, and expense of the shielding required, plus substantial eddy current loss, renders such a solution impractical.